AGL is only updated on GPS messages and not on NAVIGATION_REF or ESTIMATOR

[alonsoac]

in fw_server.ml a.agl is only updated when a GPS message is received even though the current altitude is updated when the ESTIMATOR message is received and the ground_alt is updated on NAVIGATION_REF.

This is noted specially when there is no 3D fix, so with current code agl is not updated at all.

a.agl should be updated on receipt of all these messages.

[flixr]

@gautierhattenberger, this is one for you again ;-)